1. Field of the Invention
The present invention relates to lasers. More specifically, the present invention relates to an assembly for combining the beams of a plurality of diode lasers into a spatially combined laser beam.
2. Brief Description of the Prior Art
Diode lasers, otherwise known as semiconductor lasers, are well known laser light sources for producing laser beams with a high brightness and a high beam quality. Because of these properties, diode lasers are used in many everyday applications including, for example, CD players, DVD players, laser printers, and bar-code readers.
As will be understood by those having skill in the art, a diode laser emitter generally includes a crystal structure having an n-type region, which is negatively charged, and a p-type region having a plurality of holes. The n-type and p-type regions are joined to one another in a p-n junction zone. When a charge is applied across the opposing n and p-type regions, the electrons of the n-type region combine with the holes of the p-type region in the p-n junction zone and photons, or light, is released. The photons are then channeled by the emitter to produce a laser beam.
Although diode laser emitters are very efficient at producing laser beams, the quality of the laser beam produced decreases when the power applied to the diode laser emitter is increased. Therefore, high powered laser applications—such as medical applications, material processing applications and defense applications—often require many diode laser emitters working in combination to produce a combined and more powerful laser beam with a high beam quality. These combined diode laser emitters are preferably arranged in what is commonly referred to as a diode bar, each of which typically includes anywhere from nine to fifty individual diode laser emitters arranged in a one-dimensional array.
One problem with diode bars is that they produce heat which must be conveyed away from the diode laser emitters for them to continue operating properly. To dispense this heat, diode bars are typically mounted on a base plate which functions as either a conventional convection cooled heat sink or a liquid cooled heat sink.
Another problem with diode bars is that a single diode bar is still not powerful enough for some applications. One approach to producing a high powered and spatially combined laser beam is through a vertical stack assembly. In a typical vertical stack assembly, a plurality of diode bars and heat sinks are alternately stacked one on top of another in a vertical direction. While this approach may be suitable for liquid cooled heat sinks, it is generally not suitable for convection cooled heat sinks due to a lack of surface area for dispensing heat to the air. Additionally, the stacking pitch, or the distance between the laser beams of adjacent diode bars, is dictated by the width of the heat sinks and cannot be adjusted. Large heat sinks will result in a spatially combined laser beam with a large stacking pitch, a large beam parameter product (BPP) and a low beam quality, whereas small heat sinks may improve upon these properties of the spatially combined laser beam but do not provide adequate cooling to the diode bars.
Another approach to producing a high powered and spatially combined laser beam is known as the step-and-mirror approach. A step-and-mirror assembly includes a base plate having a plurality of mounting surfaces for supporting a plurality of diode bars. The mounting surfaces are disposed at different vertical heights, and therefore, the diode bars are spaced from one another in a shelf-like manner. The diode bars are each oriented to emit laser beams in a lateral direction. A reflector (preferably a minor) is aligned with each of the diode bars for redirecting the laser beams at a 90 degree angle into a longitudinal direction, and the reflectors are laterally aligned with one another so that the laser beams from the diode bars are vertically stacked on top of one another after the redirection.
Each reflector may be rotated on two axes to adjust the far field pointing of the laser beams in the lateral and vertical directions. Additionally, the reflectors may be translated in the lateral direction to adjust the position of the re-directed beam in the lateral direction. Thus, each reflector provides three degrees of freedom for adjusting the position and far field pointing of the associated laser beam in the spatially combined laser beam.
In a step-and-mirror assembly, the vertical spacing of the steps determines the pitch of the combined laser beam, and if the steps are not spaced vertically from one another by a sufficient distance, the reflected laser beams will clip the reflectors for the other diode bars. Therefore, due to inherent tolerances in the manufacturing of the stepped base plate and the inability of the step-and-minor assembly to adjust the vertical position of the laser beams, the steps in the base plate are generally spaced vertically by a larger distance than would otherwise be ideal. Consequently, the resulting laser beam from the step-and-mirror assembly has an increased stacking pitch, increased BPP and a decreased beam quality.
The step-and-minor assembly approach to combining laser beams has additional shortcomings other than those discussed above. For example, they typically are machined into the base plate, resulting in increased manufacturing costs and an increased surface roughness, which could reduce the thermal conductivity between the diode bars and the base plate. Additionally, the thermal resistance may vary between the different diode bars due to the different material thicknesses of the base plate at the mounting surfaces. This may result in a broader spectrum of the combined beam due to a varying drift in the center wavelength between the diode bars depending on the position of each bar on the stepped base plate. Moreover, once machined, the steps cannot be adjusted to change the pitch of the combined laser beam. Finally, if coolant is used to cool the base plate, a complex and difficult to form coolant channel may be required to convey the coolant through the steps to provide adequate cooling to the diode bars.
There remains a significant and continuing need for an assembly for producing a spatially combined laser beam from a plurality of diode laser emitters with an adjustable pitch to maximize both the power and quality of the spatially combined laser beam. Further, there is a need for an assembly for producing a combined laser beam that has the same thermal resistance between each of the diode laser emitters and the base plate and can be either liquid or convectively cooled.